1. Field of the invention
The invention concerns a switchable tappet for a direct transmission of a cam lift to a tappet push rod in a valve train of an internal combustion engine, said tappet comprising a housing in whose axially extending cavity a longitudinally displaceable inner element is arranged that can be connected to the housing, in a relative position to the housing, by a coupling means, said housing comprising on an end facing away from the cavity, a contact surface for a cam, and the inner element comprising on a side of the cavity, a support for the tappet push rod, which inner element extends beyond the cavity by an extension and is biased away from the housing by at least one coiled compression spring.
2. Background of the invention
A tappet of the pre-cited type is disclosed in FIG. 2 of DE 198 44 202 which is considered to be generic art. It can be seen that a compression spring assembly extends within the housing. The compression spring assembly is supported at one end on the bottom of the housing and acts at the other end on an undersurface of the inner element. Due to the arrangement of the compression spring assembly within the housing, this latter has an unnecessary overall length. This is made up at least of the required compression path length plus the block dimension of the compression spring assembly. Due to the relatively large overall length of the housing that is made out of a relatively thick-walled tubular section, the total mass of the switchable tappet is unnecessarily increased.
It is an object of the invention to provide a switchable tappet of the pre-cited type whose total mass is decreased by simple measures.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that the coiled compressions spring is arranged at least substantially around the extension of the inner element and acts at one end on a support of the inner element facing away from the reception and at another end against an edge of the reception or on a section of the reception near the edge. Due to this xe2x80x9coutwardsxe2x80x9d shifting of the coiled compression spring, a housing can be realized that is markedly shorter than present housings. Thus, the total mass of the switchable tappet is reduced by very simple measures because all that is required within the housing now is a displacement path length for the inner element.
In a particularly advantageous embodiment of the invention, the housing has no bottom for supporting one end of the coiled compression spring in its interior, so that the housing has a completely open, tube-like configuration. This is a further step towards realizing a light-weight construction.
A particularly low-friction cam contact is obtained by making the cam-contacting surface on the housing, for example, as a roller mounted on a rolling bearing. However, a sliding contact may also be used for this purpose.
For further minimizing the overall height of the housing, and thus also the total mass, it is proposed to provide a depression of cylindrical shape (as viewed in roller direction) on the end of the inner element facing the roller. This enables the inner element in its switched-off state to xe2x80x9cplungexe2x80x9d partly over the roller.
A simple possibility for supporting the coiled compression spring that is shifted outwards is to support it on an edge of the housing or on a section of the housing near the edge. Alternatively, the reception of the housing may be given a radially stepped configuration starting from its edge so that the coiled compression spring is then supported on an annular shoulder thus formed.
A mounting of the coiled compression spring on the side of the inner element can be effected optionally through a separate stop member such as a ring, or through an annular collar formed integrally on the inner element. These measures can be implemented in a very simple and economic manner.
In a further embodiment of the invention, the extension of the inner element which is surrounded by the coiled compression spring is made as a separate and thin-walled component such as a sheet metal part. This may be clipped or snapped or fixed in another similar manner on the outer or the inner peripheral surface of the inner element. It goes without saying that in this context, a person skilled in the art would also consider other simple connecting methods like positive engagement, force-locking or fusion of materials or combinations of these.
According to another feature of the invention, the extension of the inner element is slightly funnel-shaped so as to form a lead-in aid for the tappet push rod.
Advantageously, the tappet comprises a hydraulic clearance compensation element of a type, known per se. In this way, expensive mechanical clearance adjusting measures can be dispensed with. The clearance compensation element forms a part of the inner element.
In those cases of use in which the tappet push rod has a relatively large inclination relative to the switchable tappet, a very xe2x80x9chigh-levelxe2x80x9d support must be created for the tappet push rod on the inner element. Therefore, according to a further feature of the invention, a pressure piston of the clearance compensation element on which one end of the tappet push rod is supported extends axially relatively far out of the housing so as to protrude distinctly into the extension of the inner element. It is also conceivable to advantageously configure the pressure piston so that it encloses a relatively large reservoir for hydraulic medium. This can be of advantage for a cold start of the internal combustion engine because an adequate amount of hydraulic medium is then immediately available for clearance compensation.